$\dfrac{9}{2} - \dfrac{3}{8} = {?}$
$ = {\dfrac{9 \times 4}{2 \times 4}} - {\dfrac{3 \times 1}{8 \times 1}} $ $ = {\dfrac{36}{8}} - {\dfrac{3}{8}} $ $ = \dfrac{{36} - {3}}{8} $ $ = \dfrac{33}{8}$